Transcript Continental drift

Continental drift -- One theory's journey
Today, you'd be laughed out of a geology course for
questioning whether continents move through
geologic time. But 75 years ago, only sceptics
believed that continents could take a hike. Talk about
conventional un-wisdom: W.B. Scott, former president
of the American Philosophical Society, even called the
theory of continental drift "utter damned rot." (!)
What's changed? Not the actual movement of continents,
but our understanding of geology itself. Let's take a look at
how the theory developed, and how the evidence began to
favour it.
1885: Australian geologist Edward Seuss sees similarities between
plant fossils from South America, India, Australia, Africa and
Antarctica, and coins "Gondwanaland" for a proposed ancient supercontinent with these land masses.
1910: American physicist F.B. Taylor proposes concept of continental
drift to explain formation of mountain belts.
1912-15: German meteorologist Alfred Wegener proposes theory of
continental drift, based on evidence from geology, climatology and
paleontology. Wegener names one of the ancient super-continents
"Pangea," and draws maps showing how the continents moved to
today's positions.
The main events
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225 million years ago during the Permian: One continent,
Pangea - the super continent - existed on earth. Pangea means
all lands.
200 million years ago during the Triassic: Pangea split into
Laurasia and Gondwanaland separated by the Tethys sea.
135 million years ago at the end of the Jurassic: The Atlantic
Ocean began to form as Europe and Africa split from the
Americas.
Evidence for continental drift, or plate tectonics:
Alfred Wegener
Evidence for continental drift
Matching
mountain
ranges
Structural evidence:
Mountain belts: The
Appalachian Mountains
extend to mountains in
Greenland, the United
Kingdom (Caledonian
mountains) and Norway,
indicating that these land
masses were once joined.
Stratigraphic (layers of rock) or lithological (rock type) evidence:
Rock types on opposite sides of the Atlantic are very similar, for example,
Gabon in West Africa and Brazil in South America.
Evidence for continental drift
Matching
rock types
and ages
of rocks
Continental fit: The shorelines of South America and Africa
appear to fit. This was first spotted by Sir Francis Bacon in 1620.
Geologists have looked at this further and identify continental fit
at the edge of the continental shelf as being more important. Prof.
Ballard identified the 2000 foot isobath as being where the best fit
is.
Palaeoclimatic evidence:
300 million years ago
glacial sediments and
striations suggest that the
southern continents (South
America, South Africa,
India, and South Australia)
were joined together as
Gondwana over the south
pole. Now these landmasses
are far apart.
Fossils or Palaeontologic evidence: Fossil remains of the plant
genus Glossopteris (looks like a type of fern) occur on all five
Gondwana continents. The seeds were too heavy to be carried by
wind, and would have died quickly in salt water, indicating that the
continents were once joined. Similarly, fossils of several reptiles
occur on several continents e.g. Mesosaurus and Hystrasaurus (a
non-swimming sheep size reptile). What were the arguments against
this fossil evidence?You may ask why are marsupials so rare?
More recent evidence:
Sea Floor Spreading: The ocean floors are spreading away from midoceanic ridges, indicating that the ocean floor is moving. In 1962,
Harry Hess of Princeton University proposed that seafloor spreading
would explain this movement. (Mid-oceanic ridges, more than 65,000
kilometers long, are the largest mountain range on Earth).
The age of rocks: The oldest rocks on the ocean floor are younger
than 220 million years, while the oldest terrestrial rocks are about 4
billion years old, indicating that the ocean floor is recycled back into
the Earth.
Drilling ocean floor sediments showed that they increased in thickness
away from the mid-oceanic ridges.
Paleomagnetism: British scientists find that magnetic
fields recorded in rocks from Europe and North America
indicate the rocks were formed in far different locations
than their present positions. The pattern of continental
drift recorded by rocks show Europe and North America
have drifted away from each other for more than 100
million years. This movement opened the Atlantic
Ocean.
Magnetic stripes: Earth's magnetic field periodically changes
polarity. When the hot rock solidifies at the mid-oceanic ridges, it
retains the polarity of the moment. Bands of seafloor rocks
paralleling the mid-oceanic ridges carry a record of this alternating
polarity. The bands appear to be the same on either side of the
ridge.
Magnetic stripes on the
sea-floor. Magnetic
field of Earth reverses
on semi-regular basis.
Minerals act like
compass needles and
point towards magnetic
north. “Hot” rocks
record the direction of
the magnetic field as
they cool.